Method for optical bleaching of coated papers and resultant product



United States Patent 4 Claims. (Cl. 96-82) This application is a division of United States applicat-ion Ser. No. 739,211, filed June 2, 1958; now US. Patent No. 3,047,390.

The present invention relates to a process for whitening, i.e. optical brightening, the surface color of paper, especially photographic paper, provided with one or more surface coatings composed of constituents which are not capable of strongly binding the usual optical bleaching agents, and more particularly relates to a process for brightening the white areas in the photographic image of a photographic layer in such a way that the diffusion of optical bleaching agents out of the photographic layer is inhibited.

It is generally known that the reflecting power of a paper surface can be considerably increased by applying thereto a surface coating containing finely divided fillers, which are usually pigments, having a high degree of whiteness. Examples of fillers used for this purpose are titanous dioxide, barium sulfate, zinc sulfide, calcium carbonate and silicium dioxide, among others. In this way, the surface color of the paper can be whitened to a certain extent, but not completely. If, however, these pigments are used in conjunction with substances which are slightly colored or acquire coloration in the course of further treatment or after storage, as for example where such substances either are included in the same surface coating with the pigments or are present in one or more additional surface layers, the brightening effect attributable to the pigment is nullified wholly or partially. Thus, it is seen, for instance, that when a baryta-ccated paper support is coated with a photographic emulsion, the surface color of the photographic paper is not completely white and even the whitest areas in a photographic image obtained upon such a support will still absorb a considerable amount of light. This coloration depends on various factors, such as the reflecting power of the paper used, the quality of the gelatin applied as binding agent for the photographic layers, and the kind and the amount of other compounds present such as, eg sensitizing dyestuffs added to the photographic layers during the manufacture of the light-sensitive element. This coloration can also be caused by colored decomposition products formed in the photographic layers as they are subjected to development and/or other treatment or formed in the treatment baths and absorbed by these layers as they are immersed there in. Since the cellulose fibers from which the paper is composed, as well as the gelatin used as binding agent for the photographic layers and the decomposition products which are formed during processing, absorb light particularly in the range of the short wave-lengths, the photographic paper shows in most cases a slight yellow tinge.

It has been proposed to eliminate this defect by adding to the paper support, to a surface coating and/ or to another superposed layer, dyestufls or pigments which absorb in the range of the long wave-lengths, so that the spectral character of the reflected visible light is equal to that of the incident visible light and the paper, conseuently, shows a neutral tone. Thus, it is known to add organic, light fast, blue dyestutls to a baryta-coating which Patented May 4, 1965 is applied to a paper support destined to receive a photographic emulsion layer.

As an inherent result of the latter expedient, the in cident visible light is not completely reflected but partly absorbed by the yellow paper and partly by the blue dyestutf or the blue pigment. Even under the most favorable circumstances where only an extremely small amount of dyes-tuft or pigment. is required, such as for instance, in the case of a photographic paper manufactured from a baryta-coated base stock of good quality and containing a little colored gelatin and one or more sensitizing dyestuffs of the type which during the processing can easily be removed from photographic layers, the whitest areas in the photographic image will not completely reflect the visible light.

According to a more recent proposal, substantially colorless, bluefluorescing, ultraviolet-absorbing compounds are added during the manufacture to the paper or to one or more surface coatings. These compounds absorb ultra-violet radiation of between about 3000 Angstrom units and 4000 Angstrom units and emit fluorescent light of between 4000 Angstrom units and 5500 Angstrom units and are commonly referred to in the art as optical bleaching agents. They can be added during the manufacture of a photographic paper, either to the paper support, to the layer wherein the photographic image is formed (for instance, the silver halide emulsion layer) or to a layer therebetween (for instance, a baryta-coating). The photographic element can also be treated with a solution of such compounds during processing.

The spectral partition of the reflected visible light is equalized by the use of these optical bleaching agents to that of the incident visible light by conversion of a part of the incident ultrawiolet rays to which the human eye is not sensitive into the blue light absorbed by the paper. The reflected light is now completely neutral, without loss of any part of the visible incident light. By applying this concept to the manufacture of photographic paper, not only is it possible to obtain a neutral image tone together with a strong reflecting power of the white areas in the photographic image, but photographic images to be viewed by reflection can even be manufactured which reflect more visible light than they actually received. The latter result can be achieved by the use of an exces of optical bleaching agent, so that more ultra-violet light is convented into blue light than is necessary to correct the yellowness of the photographic paper. In this way, one can change the image-tone to a colder, i.e., more bluish, tone.

In connection with the process just described, it is to be noted that if the optical bleaching agent is added to the paper alone, insuificient brightening of the images is obtained, since the Whitening eflect is annulled in large measure by the superposed baryta-coating which is present in most of the photographic papers.

It is evident that the degree of success obtained in the practice of the above-mentioned process depends heavily on the amount of optical bleaching agent which still remains in the completely finished paper. Consequently, eflorts have been made in the past to prevent the removal of optical bleaching agents, either in whole or in part, from the layers of the photographic element wherein they appear, as can happen when the element is treated in the usual treating baths.

None of the various suggestions for combating the tendency of the optical bleaching agents to wander, i.e. diffuse, out of the emulsion layer and/ or the baryta-coating has hitherto been found wholly efiective.

Summarizing, the great difliculty in finding an effective solution to the problem of loss of optical bleaching agents from photographic elements during processing is attributible mainly to the fact that optical bleaching agents are not sufliciently adsorbed on the materials, such as gelatin and baryta, of which the usual surface-coatings applied to photographic paper are composed.

It is therefore an object of the present invention to provide a method for increasing the whiteness of the surface of a paper provided with an optical bleaching agent and at least one surface coating whereby the wandering of the optical bleaching agent out of its original locus is inhibited. It is a further object of the present invention to provide a method for increasing the whiteness of photographic papers. Still further objects and advantages will appear in the following description and examples.

In accordance with the present invention, it has now been found that the wandering or diffusion of optical bleaching agents out of the layers in which they were initially incorporated can be inhibited at least substantially completely by treating certain finely divided water-insoluble macromolecular products with a solution or a dispersion of an optical bleaching agent having a pronounced substantivity of the macromolecular product used and then adding the fluorescing mass thus obtained to one or more surface coatings for the paper. In the manufacture of photographic paper, for example, the iluorescing mass can be incorporated into a layer between the paper support and the layer wherein the photographic image is formed or into the latter layer itself.

Our parent application, Serial No. 739,211, filed June 2, 1958 (now US. Patent No. 3,047,390), originally disclosed as suitable macromolecular products for the practice of the invention, certain cellulosic, polyester, polyamide and urea formaldehyde resins but was eventually restricted to the polyester subject matter. This application is directed to the cellulosic subject-matter divided out of said parent application while applications Serial No. 201,299 and No. 201,300, filed concurrently herewith, are directed, respectively, to the polyamide and ureaformaldehyde subject-matter thereof.

It is unnecessary to furnish specific examples of the preparation of cellulosic materials useful for present purposes since such material is readily available from a variety of commercial outlets. It can be derived from a number of natural sources such as wood and cotton among others, by means of any of the various purification and bleaching techniques known in the art. Insofar as we have been able to determine, neither the source nor the particular method of purification have any significant effect on the suitability of the material for use therein. The presence of substantial amounts of impurities, such as lignin and hemicellulose, as are associated with cellulose in its natural state is, of course, not desirable since these tend to contribute bad effects, especially in the case of photographic material, but the bulk of these is normally removed in the course of the usual purification methods. Any substantially pure form of strongly bleached cellulose fibers should give satisfactory performance. Products of very high purity, say containing about 90-95% or higher by weight of alpha cellulose, as can be obtained from cotton fibers and linters, or even wood by some more elaborate fiber recovery and purification procedures, may be best in some instances.

Before use, the cellulosic material is reduced to finely divided form; consequently, the condition in which it is procured is immaterial. Bales, bundles, sheets or webs (7) CH CH3 CH CH CH3 CH are all equally suitable. The subdividing operation may be carried out in any of the types of equipment useful for grinding or cutting fibrous material into small pieces. The extent of subdivision is determined primarily by practical considerations. As is necessary to be compatible with the intended purpose of the paper. Thus, in the photographic field, the major application of the invention, the particle size should be such as to permit uniform incorporation in the formulation to be coated on the paper without disrupting or otherwise disturbing the continuity or the surface quality of the finished coating. For example, particles of a size causing the coating to have a perceptibly roughened or irregular surface would be poorly adapted, if at all, for photographic materials. In general, a particle size of about to 5 microns will be acceptable with a size in the same order as that of the.

usual pigments mentioned above, say about 0.5 micron being preferred.

The following are the structural formulae of the known optical bleaching agents that have been found useful in the practice of the invention, although it will be appreciated that others having comparable chemical properties may well exist and could be substituted with generally similar effect:

SOaNa SOaNa i II Methods of preparing compounds having the above formulae are described in the following publications, the order of numbering corresponding to the list above:

(1) Chem. Abs. B44 (1950), p. 2251.

(2) US. patent specification 2,600,080.

(3) British patent specification 495,479, p. 3, No. 13; German patent specification 752,677, p. 8; Danish patent specification No. 63,688 (1945); German patent specification 972,067.

(4) British specification 624,051.

(5) F. Benber, Ber. 19 (1886), p. 3235.

(6) Prepared analogously to British spec. 624,051.

(8) Blancophor R, trademark transferred from I. G. Farbenindustrie Aktiengesellschaft to Farbenfabriken Bayer A. 'G.; Danish patent specification 63,688 (1945); German patent specification 972,067; Zeitschrift fiir die Erze-ugung von Holzstoff, Zellstotf, Papier und Pappe; Chemische Technologie der Cellulose, 8. Jahrgang, Darmstadt, April 1954, Heft 7/8; A. E. Siegrist, Die Anwendung optischer Aufhellmittel in der Papierindustrie, pp. 114-115.

(9) Neunhofer and Rosahl, Chem. Ber. 86 (1953),

(10) Org. Synth. Coll., vol. II, p. 231.

(11) US. patent specification 2,875,089, col. 6-7.

For carrying out the process according to the present invention, the desired amount of the finely divided cellulose is dispersed in an excess of Water. To this suspension is added a solution of the optical bleaching agent involved. If the optical bleaching agent is not or not sufficiently soluble, it can likewise be added in dispersed form to the suspension. Absorption of the optical bleaching agent is preferably facilitated by heating the suspension to a temperature between about 50 and 100 C., the treatment being continued until no more fluorescing substance is bound. The time required to achieve this condition will ordinarily range between about 30-60 minutes depeding upon the temperature. The mixture is then filtered under suction or centrifugation leaving a strongly fiuorescing paste which strongly retains the fluorescing substance and which can be added to the coating composition to be applied to a paper support. Thus, the fiuorescing mass can, for instance, be added to a barytacomposition which is spread upon a paper prior to applying the photographic emulsion thereto. Alternatively, paste can often be incorporated in a photographic emulsion itself. While preliminary treatment of the finely divided cellulose with the optical bleaching agent is usually best, where experience has indicated the amount of a particular agent that can be expected to be bound to a given amount of cellulose, the treatment can often be effected in situ by admixing the amount of agent and 6 amount of cellulose directly to the coating formulation. In this case, the heat applied to dry the coating will serve to promote the combination of the agent and cellulose.

The proportion of the treated cellulose needed in the coating formulation to achieve the advantages of the invention can be adjusted to meet the requirements of any particular paper and/ or surface coatings. Obviously, any amount will contribute a proportionate effect and will improve the performance of the paper to that extent. As a general rule, about 5 to 25% by weight of the coating in which the cellulose is employed, the baryta coating, for example, will suffice.

Our experiments to date have not clearly revealed the exact mechanism by which the optical bleaching agent becomes bound to the cellulose. Presumably, physical or chemical forces or a combination of the two are involved but the invention is not to be restricted to any particular theory.

The wandering of the optical bleaching agent out of the layer in which it was applied is practically completely precluded by the method of the present invention, so that the exposed photographic element after complete processing, which even may include a protracted rinsing step, yields a photographic image characterized by strongly brightened white areas.

The method of the present invention can be utilized in brightening the white areas in the developed image of the usual black-and-white and colored photographic papers as well as for whitening the surface color of any other surface-coated paper such as, for instance, papers used in duplication processes as, for example, where a positive photographic image is formed by a silver halide diffusion transfer process, or an image is printed by means of an hectographic process.

The present invention is further illustrated in the following examples without limiting, however, the scope thereof.

In the examples, the formula number given in parenthesis corresponds to the number in the list of structural formulae set forth previously.

Example I 1 kg. of ground and bleached cellulose having a particle size of about 1;]. is dispersed in 1000 cm. of water. To this suspension are added 400 cm. of a 1% aqueous solution of the compound 4,4'-bis(2-hydroxy-4-phenylamino 1,3,5 triaryl 6)-diaminestilbene-2,2-disulphonic acid disodium salt (Formula 3). By strongly stirring, the whole mass is kept in suspension at a temperature of 75 C. until no more fluorescing substance is bound by the cellulose. After this treatment the fluorescing mass is filtered under suction and the thus obtained strongly fluorescing paste is added to a conventional baryta composition to give a content of treated cellulose of 5% by weight. Then, the composition is spread upon a paper whereon one or more conventional photographic silver halide emulsion layers are to be coated. The whiteness of the surface color of the photographic paper thus treated is not lost on rinsing and the photographic element yields after exposure and complete finishing a photographic image wherein the white parts are strongly brightened.

A similar result is attained when the execution of the process of the present example, the 4,4-bis(2-hydroxy-4- phenylarnino 1,3,5-triaryl 6)-diaminostilbene-2,2'-disulphonic acid disodium salt is replaced by a same amount of 4,4'-bis(2-amino-4-hydroxyethylamino-1,3,5-triazyl--6) diamino-stilbene-2,2' disulphonic acid disodium salt (Formula 4), 4,4-diaminostilbene-2,2'-disulphonic acid (Formula 5), 4,4'-bis 2,4-di(n-butoxypropylamino)-1,3,5-triazyl-S-diaminostilbene-2,2'-disulphonic acid (Formula 6), 4,4'-bis-2,4-di(isooctylphenoxyethoxy propylamino)l,3 ,5- triazyl-6-diaminostilbene-2,2-disulphonic acid (Formula 7), or 4,4'-di(phenylureido)-stilbene-2,2'-disulphonic acid disodium salt (Formula 8).

7 Example II 1 kg. of ground and bleached cellulose is dispersed in 1000 cm. of water. To this suspension are added 400 cm. of a 1% ethanol solution of the compound 1,3-dioxo- 24butyl-6-acetylamino-1,2,3H-benz [de] isoquinoline (Formula 2). By strongly stirring, the Whole mass is kept in suspension at a temperature of 50100 C. until no more fluorescing substance is bound by the cellulose. After this treatment, the fluorescing mass is filtered and further treated as indicated in Example I.

A similar result is attained when on carrying out the process of the present example, the 4-acetylamino-1,8- butylnaphthylimide is replaced by a same quantity of 1,3- diphenyl-S-(p-iodophenyl)-pyrasoline (Formula 9).

Example III 1 kg. of ground cellulose is dispersed in 1000 em. of water, together with 10 g. of 1,2-di(S-methylbenzirnidazyl- 2)-ethylene (Formula 1). By strongly stirring the whole mass is kept in suspension for 30 minutes at a temperature of 50-100 C. After this treatment the fluorescing mass is sucked off and further treated as given in Example I.

A similar result is attained when on carrying out the process of the present example, the 1,2-di(-methy1- benzimidazyl-Z)-ethylene is replaced by a same amount of 1,2-di-(benzimidazyl-2)-ethylene (Formula or 1,2- di(S-methylbenzoxazoyl-Z)-ethylene (Formula 11).

In the above examples, the cellulose was of the type used for the manufacture of cellulose triacetate and had a content of alpha cellulose of 97 to 99% by weight.

We claim:

1. A light-sensitive photographic paper comprising a paper support, a white pigmented coating on a surface of said support, and a superposed light-sensitive silver halide emulsion layer, said white pigmented coating containing discrete fluorescent particles obtained by heating an aqueous suspension of finely divided cellulose through which suspension is uniformly distributed an optical brightening agent selected from the group consisting of:

(1) 1,2-bis-[2-(5-1nethyl-benzimidazolyl)]-ethylene,

(2) 1,3-dioxo-2-butyl 6-acetylamino-2,3 dihydro-1H- benz[de]isoquinoline,

(3) 2,2'-di-sulfo-4,4-bis-[2-(4-hydroxy-6-anilino)-s-triazinyl1-stilbene di-sodium salt,

(4) 2,2-di-sulfo-4,4'-bis-[2-(4-amino 6-hydroxyethylamino) -s-triazinyl]-stiibene di-sodium salt,

( 5) 2,2-di-sulfo-4,4-di-amino-stilbene,

(6) 2,2-disulfo-4,4-bis-{2-[4,6-di-(w-n-butoxypropylamino-) =s-triazinyl] }-stilbene,

(7) 2,2'-di-sulfo-4,4-bis-{2-[4,6-di-(p-1,1,3,3-tetramethyl-butyl-phenoxy-ethoxy-propylamino-)]-s-triazinyl stilbene,

(8) 2,2-di-sulfo-4,4-di- (phenylureido- -stilbene disodium salt,

(9) 1,3,5-tri-phenyl-A apyrazoline,

( 10) 1,2-bis- [2-benzimidazolyl] -ethylene,

( 1 1) 1,2-bis-[2-(S-methyl-benzoxazolyl] -ethylene.

2. A paper according to claim 1 wherein said fluorescent mass constituted about 5 to by weight of said pigmented coating.

3. A paper as in claim 1 wherein said cellulose is at least about by weight alpha cellulose.

4. A paper as in claim 1 wherein said cellulose has a r particle size not greater than about 1 micron. 0

References Cited by the Examiner UNITED STATES PATENTS 2,332,038 10/43 Wynd et al. 9682 2,639,990 5/53 Kendall et a1. 9682 2,723,197 11/55 Libby et a1 9694 2,822,288 2/58 Harvey et al. 117-33.5 2,846,397 8/58 Ackermann 260249.5 3,031,326 4/62 Fleck 117-335 3,043,710 7/62 Patten et al 11733.5 3,047,390 7/62 Koerber et a1 96---82 FOREIGN PATENTS 573,462 4/59 Canada. 878,823 2/43 France.

NORMAN G. TORCHIN, Primary Examiner.

Patent No. 3,181,948 May 4, 1965 Willem Karel Koerber et al It is hereby certified that er ent reqiiring correction and that t oorrectedbelow.

ror appears in the above numbered pathe said Letters Patent should read as In the heading to tl 1e printed specification, after line 11 insert the following:

Claims priority, application Netherlands, June 6, 1957, 21 ,900

Signed and sealed this 30th day of November .1965,

(SEAL) iltest:

ERNEST W. SWIDER EDWARD J BRENNER Iii-sting Officer Commissioner of Patents 

1. A LIGHT-SENSITIVE PHOTOGRAPHIC PAPER COMPRISING A PAPER SUPPORT, A WHITE PIGMENTED COATING ON A SURFACE OF SAID SUPPORT, AND A SUPERPOSED LIGHT-SENSITIVE SILVER HALIDE EMULSION LAYER, SAID WHITE PITMENTED COATING CONTAINING DISCRETE FLUORESCENT PARTICLES OBTAINED BY HEATING AN AQUEOUS SUSPENSION OF FINELY DIVIDED CELLULOSE THROUGH WHICH SUSPENSION IS UNIFORMLY DISTRIBUTED AN OPTICAL BRIGHTENING AGENT SELECTED FROM THE GROUP CONSISTING OF: 